The lightbox is nice and is perfect for the screenshots, but it should not be added to the main image. The main image should still link to the demo. Most WP themes don’t use screenshots either so it’s very redundant – http://cl.ly/image/1I2h3D2c0X2g

Also the screenshots button/link should ONLY display if an item actually has screenshots. If an item only has a main image uploaded then it’s just useless bloat. You can either make the live preview button longer or add a different button instead (download/support).